The conductivity of an electrolyte depends on the relative concentration of the water. At low humidity concentrations, it is so that the conductivity increases with the water content. If the electrolyte is heated in dependence on its conductivity, part of its water content evaporates. This process continues until the electrolyte is waterless, unless the electrolyte can absorb water vapor from the surroundings. If the electrolyte is surrounded by a mixture of water vapor and gases, ultimately an equilibrium between the amount of water vapor given off and the amount of water vapor taken up establishes itself. This equilibrium is characterized by a certain temperature of the electrolyte. This temperature is a direct measure of the water-vapor partial pressure in the mixture of water vapor and gases.
An apparatus for measuring water-vapor partial pressure having the features described at the beginning is known from German Offenlegungsschrift 2,416,179. There, the heater is in direct thermal contact with the electrolyte and consequently also with the measuring sensor. The electrolyte is therefore heated by heat conduction from the heater. In order to be able to transfer heating output by means of heat conduction, temperature gradients are necessary. In the case of the apparatus according to German Offenlegungsschrift 2,416,179, these temperature gradients lead to undesired effects. The temperature measured as equilibrium temperature is a temperature which actually is only applicable to the heater and not to the electrolyte. The temperature gradient causes a thermotransport in the electrolyte, so that there is no longer homogeneous, and consequently defined, distribution of the electrolyte. The overall equilibrium which has to be achieved for measuring the water-vapor partial pressure also requires that the mixture surrounding the electrolyte is at the equilibrium temperature. Since this mixture is in this case heated indirectly by means of the electrolyte alone, there is only a very small boundary layer of the mixture which is also in equilibrium. All these effects have the result that the apparatus according to German Offenlegungsschrift 2,416,179 proves to be technically complex in terms of control. Even if only slightly maladjusted, it exhibits an unsatisfactory transient response. The state of equilibrium is only achieved after long periods or not at all.
German Patent 1,598,993 discloses an apparatus for measuring water-vapor partial pressure which differs from the type described at the beginning in that the measuring sensor is not arranged in direct thermal contact with the electrolyte. The heater is also neither in direct thermal contact with the electrolyte nor with the measuring sensor. The thermal contact between the measuring sensor, the electrolyte and the heater is brought about by the mixture of water vapor and gases. In order to obtain the equilibrium temperature at all points of the measuring apparatus, a motor is provided for forced circulation of the mixture. The heating of the electrolyte is consequently performed indirectly. The heater heats the mixture, the motor circulates the mixture, the latter comes into the proximity of the electrolyte and there gives off again part of the amount of heat taken up. The comparison of the temperature between the electrolyte and the measuring sensor takes place similarly. In order to achieve a homogeneous temperature distribution within the apparatus, it is necessary in the case of an apparatus according to German Patent 1,598,993 to perform a turbulent forced circulation of the mixture. Otherwise, the temperature differences between the mixture and the electrolyte become great. These temperature differences are then, however, also likely to exist between the electrolyte and the measuring sensor and are thus to be assumed as a measure of the inaccuracy of the measured water-vapor partial pressure. The turbulent forced circulation of the mixture also results in the heater itself not being able to be at a significantly higher temperature than the other component parts of the apparatus. Consequently, heat transfer from the heater to the electrolyte takes place exclusively via the mixture, i.e. by means of convection. The turbulent forced circulation of the mixture entails very serious disadvantages. The necessary motor reduces the reliability of the apparatus. Due to the air flows at the surface of the electrolyte, the latter is eroded. The reaction rate between aggressive agents contained in the mixture and the electrolyte is greatly increased, since the maximum concentration of aggressive agents is always on the surface of the electrolyte. In the case of a static mixture, the electrolyte reacts only with the quantities of aggressive agents in its vicinity, the reaction rate decreasing with time. The quantities of aggressive agents in the direct vicinity of the electrolyte are consumed. Further aggressive agent can diffuse to the surface of the electrolyte only slowly. The electrolyte is also carried away directly by the turbulent forced circulation of the mixture. Small droplets or crystallites are entrained at the surface by the mixture flowing past. The electrolyte is thus distributed throughout the entire apparatus. This has the result that the cell has to be frequently charged with the electrolyte. All of this stands in the way of using the apparatus according to German Patent 1,598,993 for long-term measurements. Apparatuses for measuring watervapor partial pressure are used predominantly for meteorological purposes. Frequently they operate in weather huts which are checked only at intervals of several months.
It is known from both of the abovementioned publications to use lithium chloride as electrolyte for the electrolytic cell. The construction of the electrolytic cell is also disclosed there. The cell has a tubular fabric impregnated with electrolyte, on the surface of which fabric a pair of wire electrodes of noble metal is wound in bifilar and equidistant fashion.